Depends on what you want it to do. I have a 100 gal rubbermaid under my 100 gal tank as a pre-sump that feeds into a 50 gal barrel sump before returning water to the tank. Overkill you say? maybe. But if you're going to keep big eaters and want water stability it's great, you can safely store heaters, and media out of sight, or if your fish eat plants if you want a place for them as a nitrate filter, or even have a compartment for fry, or a fish that's getting beat up and needs a rest.